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KHV: LATENT CARRIERS - What Hobbyists Need To Understand

Outbreaks of KHV over the past 18 months or so, and particularly in the early months of this year, have involved some of the leading Japanese breeders and several well-known U.S. koi dealers. Understandably, a number of serious hobbyists have become nervous about the situation. There are concerns that breeders and dealers are not taking their bio-security as seriously as they should, perhaps becoming lax after several years without outbreaks among the well-known names. I have voiced that concern myself, although the U.S. dealer outbreaks with which I am most familiar either involved circumstances where the dealer had no choice but to mix fish, or it is understandable why the dealer thought a normal protocol step could be skipped. Even when bio-security is a top priority, things can happen.

One of the things that can happen is a breeder (or dealer) can have a carrier koi and not know it. The existing tests for KHV focus on antibodies produced by an exposed koi. Many believe that if they test all of their koi and all the tests come back negative, the koi is KHV-free. That is not necessarily true. Think about some of the vaccinations you have gotten. Over time the effectiveness wears off and it becomes time for another vaccination. Over time the antibodies generated in response to the vaccination cease to be produced by the body and eventually are no longer present in sufficient degree to provide any immunity. Similarly, the antibodies produced by a koi in response to a KHV infection cease to be detectable over time. How long it takes before a survivor no longer has sufficient antibodies present to be detectable is a matter for continued in-depth study. However, indications are that it is as short as one year, which is the time period to assume until more is known. It likely varies from fish to fish, and perhaps also from one KHV strain to another. What percentage of survivors are non-detectable based on antibody tests after two years? after three years? Nobody knows, but likely a lot.

What surprises a lot of folks is learning that although a survivor no longer has detectable antibodies, and tests negative, it is still a carrier. It has been known since a study done at Oregon State in 2011 that the KHV virus DNA can be found in the lymphocytes (white blood cells) of exposed fish. There is currently no commercially available test for detecting this DNA, but there is one in research. It will hopefully become available sooner rather than later, and at a cost that makes it feasible for wide-spread use.

These "KHV-negative carriers" may not transmit the disease for years, and perhaps some never. A koi can be stressed without an infection breaking. It can be stressed again, and a third time, all without breaking. Then at some point, it does break out and every koi in the same water becomes infected. Until a lot more is known, including about the specific strains of KHV, there will be far more questions than answers. In the meantime, the risk can be minimized through bio-security measures and proper use of the available tests. But, there is no means currently available to be 100% sure. Hopefully the white cell DNA test will provide such certainty and become available to all. In the meantime, it is somewhat unfair to assume a breeder who suffers an outbreak has ignored bio-security. The latent carrier that tested negative could have been the culprit.

Spike Cover put together a good summary of what is known about KHV last year. I will post it next.

The following was authored by Spike Cover about 6 months ago. It was as up to date as anything at that time.

Current Facts About KHV

It is very contagious to Koi and common carp.
KHV can and does establish latency in Koi and common carp.
There is no cure for KHV disease or latency. That said, active disease can often be arrested by raising the temperature of the fish to ≥ 86°F and holding them at this temp. for several days. Survivors will almost certainly be carriers.
Latency is apparently wide-spread in domestic Koi, food carp, feral Koi and wild common carp.
Latency appears to be possible w/o the requirement for full-blown disease to have occurred. This means that carriers can show up in stocks that have not experienced a disease outbreak.
There is no commercially available vaccine for KHV
Other species can carry the virus and can shed viral particles that can infect Koi and common carp but those other species do not exhibit clinical signs of the disease
Only active KHV disease is easily identifiable. It is diagnosed by clinical signs and a positive PCR test on gill tissue, a vent or gill swab or on a fecal sample.
The viral DNA of latent carriers can be detected by what is currently a research-only test on lymphocytes (specific blood white cells).
Latency can be inferred by testing Koi blood for anti-KHV antibodies or for KHV virus neutralization properties. Anti-KHV antibodies can be detected in Koi for up to one year post infection. There is a commercially available test for this done by UC Davis. Virus neutralization testing can be done at the Univ. GA, Athens and possibly others.
Stress can sometimes cause latent carriers to break with active disease. Thus carriers can sometimes be identified by stressing and observing them (for several weeks) for signs of the disease. If such signs appear, a proper PCR test can confirm (or rule out) the presence of the disease.
What can customers reasonably expect from Koi dealers?
Dealers should offer only Koi to their customers that are free of active KHV disease.
Dealers cannot screen fish definitively for latency, i.e., for the presence of viral DNA in the fish, as a commercially available test for such latency is not currently commercially available.
Stressing fish and observing them for subsequent signs of KHV disease is one way to possibly uncover latent carriers. However no specific level of stressing has been determined to be adequate to uncover all carriers. This technique is the only somewhat economical method currently known to be even partially successful for screening fish on a large scale. Shipping stress has been shown to be significant enough to sometimes (maybe often) induce latent carriers to break with the disease. Unfortunately, this is not always the case.
Dealers can sample Koi from various sources for anti-KHV antibodies and, if that test is positive, they can infer that KHV latency exists within that population. This requires that dealers keep Koi from various sources separated. Adequate sampling to detect most carriers requires numerous samples and would likely be costly and likely economically prohibitive unless the population size was large enough to spread the cost of testing over many fish. **** Individual fish can be tested for the presence of anti-KHV antibodies for greater confidence relative to those specific fish. This would likely apply to more expensive fish.

What can hobbyists do to protect themselves?
Understand the disease and what is possible and not possible relative to identifying diseased fish and latent carriers.
Understand what the dealer(s) from whom they purchase fish do to screen for KHV disease and latent carriers.
Buy only from dealers that practice good biosecurity, quarantine and disease screening.
Practice good biosecurity and quarantine at home. This includes quarantining all new Koi and those returning from shows or other trips outside the hobbyist’s control.

It is a tough topic... in every way. The science is difficult to pull together without monitoring journals worldwide, and it is tough on everyone in the hobby, whether they have an outbreak or not.

As the 'white cell latency' test gets used, I expect we will find that there are a lot of latent carriers all around us, and that many of them live a long life without spreading the disease... until some do. What I just said may come to be seen as fearmongering, or simply realism. Right now, there is not enough known to say.

This guarantees there are no latent carriers and koi can be produced from this stock in bio-secure facilities with confidence of KHV-free offspring. Of course, there can never be new breeding stock utilized, or all new breeding stock have to be quarantined and be tested likewise, in order for assure continued KHV-free facilities.

This is a huge step forward in protecting the hobby and giving the level of confidence hobbyists want. But, it is not an easy thing to do and there is considerable expense. The test is only available by participating in a research program working toward making an affordable test generally available. So, right now it would be unfair to demand like testing by other breeders. We will have to wait for the test to become more available. And, it will probably be quite some time before it is feasible for dealers to test every fish, if ever. Even a comparatively low cost would make inexpensive tosai rather costly. But, it is seeming more likely that in a few years it will be possible for breeders to have 100% confidence there are no latent carriers in their breeding stock. When that becomes the accepted foundation for bio-security, the elimination of KHV from the hobby will be possible. Like much that is possible, however, that 'perfect day' will not happen until the test is readily available, economically feasible and educated consumers direct their patronage to those who have taken the extra step to be truly KHV-free. .....and all the latent carriers already spread around in ponds have passed away. It has been about 20 years since the first outbreaks. Maybe another 20 to get to a 'worry-free' state?

Mostly I hear about quarantine, detection, prevention, and a bit of treatment, however, I rarely see a discussion on eratication. And perhaps I am not in the circle of information, but I rarely, if ever, hear about what the source is doing for eradication. It seems like the minimum is being done, not the maximum. Which begs the question- Is complete eradication possible? Or is simply to tame and control?

KHV is now present in wild populations around the globe, with the possible exception of Australia (unless they went forward with the idea of introducing it to control feral carp). It is not limited to carp. Studies of eastern European fish populations indicate it is now carried by numerous other species. There are die-offs in natural water bodies every year. They do not get reported as widely as they were a decade ago. So, I think eradication is not feasible. It is also prevalent in breeding populations of koi in southeast Asia and Indonesia, which supply very cheap tosai to dealers around the world... avoided by the well-known dealers in the U.S. and Europe, but still in circulation. (Watch out for the goldfish dealer who brings in a shipment of cheap tosai from the southeast Asia goldfish breeding centers. )

BUT, eradication within a closed system of koi breeders could occur. For it to happen, the first step is elimination of all latent carriers. The white blood cell testing for KHV DNA gives hope this can be accomplished. BUT, even if the test becomes available and economically feasible, there has to be uniform application by breeders in the 'closed system', with bio-security measures including steps assuring exclusion of all koi outside that system. Due to its culture and position in the koi hobby, Japan is more likely to be able to establish such a closed system than elsewhere. The Japanese breeders have gotten close to the closed system I envision, but the recent outbreaks show they have not reached the point of 100% closure. They cannot get there until all latent carriers are eliminated within their circle. Even with all of that... so many 'buts'.... look at the current measles outbreak in the U.S. About two decades ago, measles was declared eradicated in the U.S. People get lax when they feel safe. When the subject is ornamental fish, how much more lax than when it is their own children's health?

I think we can get to a point where the koi hobbyist has an extremely low risk, as long as they stay within the circle of well-known dealers and breeders. The comparatively low level of KHV outbreaks among the more serious hobbyists today shows that the existing testing measures and bio-security practices have done a world of good. We are much better off than we were. It can be better still. BUT (another one), education within the circle of key participants is required. A discussion on another board suggests that those within that circle, although better informed and knowledgeable than the norm, are not fully up to date and do not fully comprehend the latency time bomb.